Push-pull based voltage-clamping electronic ballast

ABSTRACT

An electronic ballast that includes a DC power source and a transformer comprising first, second and third windings. The first, second and third windings being inductively coupled, and the third winding is connected in parallel with a load. The ballast also includes first and second circuit pathways connected in parallel. The first circuit pathway comprises a first switch connected in series with the first winding, and the second circuit pathway comprises the second winding connected in series with a second switch. The DC power source is connected in parallel with the first and second circuit paths to provide an input voltage source. A capacitor connects the point between the first switch and first winding of the first circuit pathway with the point between the second switch and the second winding of the second circuit pathway.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to electronic ballasts, and more particularly to electronic ballasts that clamp a voltage across a switch and recycle leakage energy of a transformer such that a power converter has high power conversion efficiency with no voltage spike in the switch.

[0003] 2. Description of the Related Art

[0004] Traditional ballasts for use in fluorescent lamps consisted of large and heavy magnetic coils. These have been replaced by electronic ballasts that are compact and light. A common device used as an electronic ballast is a self-oscillating, push-pull converter. One well-known push-pull converter is shown in FIG. 1. As shown in FIG. 1, the converter consists of a DC power source V_(in), switches S₁ and S₂, and transformer T. The switches S₁ and S₂ are each a standard metal-oxide semiconductor, field-effect transistor (MOSFET). D_(s1) and C_(s1) represent body diode and internal capacitance, respectively, of switch S_(1,) and D_(s2) and C_(s2) represent body diode and internal capacitance, respectively, of switch S₂. Transformer T contains three windings, N1, N2 and N3, and is designed such that a current present in either winding N1 or N2 produces a current in winding N3 to drive the load Ro, typically a fluorescent lamp. Inductor L_(r) and capacitor C_(r) form a standard resonant tank to provide high frequency voltage to the load Ro. Three current paths flowing in the circuit are represented as having currents i_(s1), i_(s2) and i_(Lr). Voltage V₀ is shown across load Ro.

[0005]FIG. 2 is a graph of the waveforms associated with the conventional push-pull electronic ballast circuit of FIG. 1. As shown in FIG. 2 and also found in other like electronic ballasts is a phenomenon of overvoltage or high voltage spikes that occur across the switches when the device is switched off. For example, at t₀ when S₁ is switched off a spike is shown in FIG. 2 to occur in the voltage across S₁, i.e. V_(ds1). The principle reason for the voltage spike is that the opening of switch S₁ creates an abrupt interruption (discontinuity) in the current through N₁. There is an release of stored energy due to a leakage inductance associated with winding N₁ (represented as L_(k1) in FIG. 1), thereby inducing a current that charges C_(S1) and consequently creating a high voltage spike across switch S₁.

[0006] The same voltage spike characteristic occurs in the voltage across S₂, i.e. V_(ds2), at time t₂ when S₂ is switched off The voltage spike is caused by transient or leakage inductance L_(k2) associated with transformer winding N₂. The circuit of FIG. 1 provides no avenue for the released energy of the winding to flow other than the internal capacitance of the adjacent switch, which results in the voltage spike. In the prior art the leakage inductances are always present to some extent and there is always a danger of inducing these high voltage transients when switching off. The faster the switching the greater the voltage spikes. An excessive voltage spike will result in permanent damage to the switching device such as a burn through of the semiconductor layers.

[0007] One method of reducing the voltage spikes is to include a “snubber” circuit comprised of an additional diode, capacitor and resistor, connected in parallel with the switch (such as S1 and S2). While the snubber circuit can limit the peak voltage of the spike, it slows down the effective switching speed of the circuit, and in doing so it absorbs energy that is dissipated as heat, thus reducing the overall power conversion efficiency of the ballast.

[0008] Another method of reducing the voltage spikes is to “clamp” the voltage across the switch (such as S1 and S2). This is accomplished by two conventional methods. A first method connects a zener diode across the switch. As the voltage spike occurs the Zener diode turns on allowing the current to flow through, thus reducing or eliminating the voltage spike across the switch. A second method connects a diode in series with a with a parallel capacitor and resistor network across the switch. The capacitor charges to a constant voltage through a current flow, thus absorbing the voltage spikes. The resistor dissipates the stray inductances while maintaining the voltage across the capacitor. As with any clamping circuit, stray circuit inductances exist, and voltage spikes will be produced. Also, a diode by its nature does not provide an instantaneous clamping action.

SUMMARY OF THE INVENTION

[0009] It is therefore an aspect of the present invention to provide an electronic ballast that clamps the voltage across the main switch and eliminates voltage spikes due to the leakage inductance of the transformer.

[0010] It is another aspect of the present invention to provide an electronic ballast that recycles the leakage energy of the transformer to improve the power consumption efficiency of the circuit.

[0011] These and other aspects of the present invention are achieved by providing an electronic ballast that incorporates a clamping capacitor therein in such a manner that the overall circuit eliminates voltage spikes across the switches and recycles the transient leakage inductances of the transformer.

[0012] Thus, the invention comprises an electronic ballast that includes a DC power source and a transformer comprising first, second and third windings. The first, second and third windings being inductively coupled, and the third winding is connected in parallel with a load. The ballast also includes first and second circuit pathways connected in parallel. The first circuit pathway comprises a first switch connected in series with the first winding, and the second circuit pathway comprises the second winding connected in series with a second switch. The DC power source is connected in parallel with the first and second circuit paths to provide an input voltage source. A capacitor connects the point between the first switch and first winding of the first circuit pathway with the point between the second switch and the second winding of the second circuit pathway.

[0013] The first and second switches may be transistors, for example, metal-oxide semiconductor field-effect transistors. The first switch and the second switch are electrically connected to a digital controller or some other switching source that provides switching signals to the first and second switches. The first and second switches are switched off and on in a repeating cycle, the first switch switched on while the second switch is switched off and the first switch switched off while the second switch is switched on. When the first switch is switched off, a current in the first winding provides a charging of the capacitor. Likewise, when the second switch is switched off, a current in the second winding provides a charging of the capacitor. The voltage across the first and second switches is substantially constant over the interval that the respective switch is turned off.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] The above and other aspects, features and advantages of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings in which:

[0015]FIG. 1 is a circuit diagram of a conventional push-pull electronic ballast;

[0016]FIG. 2 is a graph of typical switching waveforms of the circuit of FIG. 1;

[0017]FIG. 3 is a circuit diagram of a push-pull based voltage clamping electronic ballast according to an embodiment of the present invention;

[0018]FIG. 4 is a graph of switching waveforms of the circuit of FIG. 3;

[0019]FIG. 5 is a circuit diagram representing the circuit of FIG. 3 when switch S₁ is on; and

[0020]FIG. 6 is a circuit diagram representing the circuit of FIG. 3 when switch S₂ is on.

DETAILED DESCRIPTION OF INVENTION

[0021] Preferred embodiments of the present invention will be described herein below with reference to the accompanying drawings. In the following description, well-known functions or constructions are not described in detail since they would obscure the invention in unnecessary detail.

[0022]FIG. 3 is a circuit diagram of a preferred embodiment of the present invention. A DC power source V_(in) supplies DC power to the circuit. L_(k1) and L_(k2) represent the leakage inductances of windings N₁, N₂ of transformer T. D_(s1) represents body diode of switch S₁. D_(s2) represents body diode of switch S₂. C_(S) is a clamping capacitor that, as described farther below, acts to absorb and recycle leakage energy of transformer T. Transformer T has three coupled windings, primary windings N₁ and N₂ and secondary winding N₃. In the preferred embodiment, the number of windings of N₁ is equal to the number of windings of N₂. The normal current paths through switches S₁ and S₂ and windings N₁ and N₂ are represented by i_(s1), i_(s2), i_(n1) and i_(n2)x, respectively, having the sign conventions as shown. The voltages across switches S₁ and S₂ and capacitor C_(s) are represented by V_(s1), V_(s2) and V_(C), respectively, having the sign conventions as shown. Inductor L_(r) and capacitor C_(r) form a resonant tank and resonant tank current i_(Lr) induced in N₃, provides a high frequency voltage V₀ across the load R₀, i.e. a fluorescent lamp. The operation of FIG. 3 will be described with reference to FIG. 4.

[0023]FIG. 4 is a graph of the various waveforms associated with the switching of the circuit of FIG. 3 during a steady state switching. Waveforms i and ii give the relative switching cycles of S₁ and S₂, respectively. Just prior to t₀, switch S₂ is conducting and carries its maximum current (see waveform vi) at t₀, which is equal to i_(n2)-i_(n1). Since switch S₂ has been on for the duration of a switching cycle at t₀, the current i_(n2) through N₂ due to V_(in) has increased to a maximum (see waveform viii), while capacitor C_(S) is discharging a voltage (described further below), thus creating a negative current i_(n1)(see waveform vii). By the sign convention of i_(n1) shown in FIG. 3, both of these currents add to i_(s2).

[0024] During the time period t₀ to t₂, switch S₁ is conducting or on and switch S₂ is off During t₂ to t₄ switch S₂ is on and switch S₁ is off (see waveforms i and ii of FIG. 4). It is noted that a switch is also considered “on” where the associated body diode is conducting. Thus, for example, waveform v at t₀ shows that i_(s1) has a negative value even though waveform i shows that switch S₁ remains off for a brief period after t₀. The negative current flow in this interval is through D_(s1).

[0025]FIG. 5 is the equivalent circuit of FIG. 3 during time period t₀ to t₂, when S₁ is on and S₂ is off. (For ease of description, the leakage inductances L_(k1), L_(k2) associated with windings N₁ and N₂ are omitted from the description of the basic operation of FIG. 5 (and FIG. 6 below); the impact of the leakage inductance at switch off will be considered in further detail below.) As shown in FIG. 5, the input source voltage V_(in) is applied to winding N₁. Since winding N₁ is coupled to winding N₂ the increasing current i_(n1) created by V_(in) induces a voltage and a resulting current i_(n2) in N₂. Since the number of windings of N₁ and N₂ are equal (have a turns ratio of 1:1), the induced voltage across N₂ is equal to (N₂/N₁)*V_(in), and thus is equal to V_(in). By the dot convention of N₁ and N₂, the voltage drop is across N₂ in the upward direction in FIG. 5 (i.e., in the direction of point Z to X). Because point X in the circuit has a voltage level V_(in), the voltage at point Z must be 2V_(in). Because the voltage at point Y also has voltage level V_(in), the voltage drop from Z to Y is equal to V_(in). Thus, capacitor voltage V_(cs)=V_(in).

[0026] Because of the coupling between N₂ and N₁, current i_(n2) is equivalent to −i_(n1), with a slight DC bias voltage offset due to energy absorbed from V_(in) as shown in waveforms vii and viii of FIG. 4. The current i_(cs), of capacitor C_(S) must equal the current i_(n2) flowing through N₂, as shown in waveforms viii and ix. Thus, from time t₀ to t₁ the current i_(cs) acts to charge capacitor C_(s), whereas from time t₁ to t₂ the current i_(cs) acts to discharge C_(s).

[0027] As noted, the voltage at point Z of the circuit of FIG. 5 is equal to 2V_(in). This is also the voltage drop across the open switch S₂ in the time interval t₀ to t₂, as shown in waveform iv of FIG. 5. Focusing again on t₀ of FIG. 4, it is seen from waveforms ii and iv that switch S₂ does not have the voltage spike when switched from on to off, as in the prior art devices. Instead, waveform iv shows that the voltage across S₂ rises directly to a value of 2V_(in) and is maintained at that level for the duration t₀ to t₂. When S₂ is turned off at t₀, the current i_(n2) flowing in N₂ is immediately shunted to the circuit loop that includes capacitor C_(s), thus immediately charging C_(s) with the current i_(n2). Thus, any current contributed to i_(n2) by a release of magnetic energy stored in leakage inductor L_(k2) associated with N₂ at t₀ acts to charge C_(s) from t₀ to t₁. Thus, there is no mechanism in the circuit that creates a voltage spike across S₂.

[0028] It is again observed that the discharge of capacitor C_(s) from t₁ to t₂ creates a negative current in _(cs) (which is equivalent to i_(n2)) as shown in FIG. 5 by the sign convention. The current thus flows counter-clockwise around the loop including C_(s), N₂ and S₁, thus contributing (along with the current i_(n1) through N₁ created by V_(in)) to the relatively large positive current i_(s1) through switch S₁ from t₁ to t₂.

[0029] By symmetry, analogous description applies during the portion of the switching cycle when S₁ is off and S₂ is on during t₂ to t₄, as shown in FIG. 6. In short, supply voltage V_(in) is applied across N₂ (including associated leakage inductor L_(k2)). Thus, current i_(n2), which now flows through closed switch S₂, begins to build from a negative value at t₂ to a positive value at t₄. By the coupling of N₂ and N₁, voltage V_(in) is induced across N₁ in the upward direction (by the dot convention). This implies that the voltage at point Z′ is −V_(in), V_(C)=V_(in) and V_(s1)=V_(in)−(−V_(in))=2V_(in), as shown in waveform iii of FIG. 4.

[0030] Input voltage V_(in) applied to N₂ acts to reverse current i_(n2), as shown in waveform viii of FIG. 4. A closed circuit is also formed by the loop including C_(S), N₁ and S₂; thus i_(n1) and i_(cs) are equal from t₂ to t₄, as shown in waveforms vii and ix of FIG. 4. Currents i_(n1) and i_(cs) are positive from t₂ to t₃, thus charging capacitor C_(S) as the current in N₁ is reduced. As described above, this shunting of the current to C_(s) avoids a voltage spike across S₁ at t₂. (See waveform iii at t₂). The combination of current i_(n1) (or equivalently, i_(cs)) and current i_(n2) through switch S₂ from t₂ to t₄ results in a relatively large current through S₂ at the end of the cycle t₄. (See waveforms vii, ix and vi at t₂ to t₄.)

[0031] Subsequent switching cycles are analogously described. Thus, the inventive circuit avoids the voltage spikes across the switches when a switching off occurs.

[0032] The winding currents i_(n1) and i_(n2) through N₁ and N₂ are always continuous sinusoidal waves as long as the switching frequency is chosen close to the resonant frequency, which is determined by the values of resonant inductor L_(r) and resonant capacitor C_(r). In any operation mode, the winding currents are always continuous and the switch voltages are always clamped up to two times the input voltage. Actually, the leakage inductances of transformer T can be used as part of the resonant inductance. In any operation mode, there are two sub circuits operating in parallel. There is no source within the circuit that generates the high voltage spike since the current flowing through windings N₁ and N₂ is always continuous. The leakage energy is thus recycled.

[0033] Given again that the number of windings of N₁ equals those of N₂, the maximum voltage across switches S₁ and S₂ is equal to twice the input voltage V_(in). Compared with the conventional push-pull resonant inverter, in the present invention the windings N₁ and N₂ carry continuous current and work together at all times to produce the resonant tank current i_(Lr), and the voltages across the switches S₁ and S₂ are always equal to twice the input voltage V_(in). As a result, the present invention has a high efficiency and low voltage stress. And due to the fact that the present invention recycles the leakage inductances, there is no requirement for the leakage inductances of the transformer to be taken into effect during design.

[0034] It will be understood that the invention is not limited to a particular type of switching device, and that other switching devices such as bipolar-junction transistors (BJTs) or junction field-effect transistors (JFETs) can be used.

[0035] While the invention has been shown and described with reference to certain preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims. 

What is claimed is:
 1. An electronic ballast, comprising: a DC power source; a transformer comprising first, second and third windings, said first, second and third windings being inductively coupled, the third winding being connected in parallel with a load; first and second circuit pathways connected in parallel, the first circuit pathway comprising a first switch connected in series with the first winding, and the second circuit pathway comprising the second winding connected in series with a second switch, the DC power source also connected in parallel with the first and second circuit paths to provide an input voltage source; a capacitor connecting the point between the first switch and first winding of the first circuit pathway with the point between the second switch and the second winding of the second circuit pathway.
 2. The electronic ballast of claim 1, further comprising: a first diode connected in parallel with said first switch; and a second diode connected in parallel with said second switch.
 3. The electronic ballast of claim 1, wherein said load is a resonant circuit comprising a fluorescent lamp.
 4. The electronic ballast of claim 3, wherein said resonant circuit is comprised of an inductor in series with a capacitor and a fluorescent lamp, said capacitor and said fluorescent lamp being in parallel with each other.
 5. The electronic ballast of claim 1, wherein said first and second switches are transistors.
 6. The electronic ballast of claim 5, wherein said transistors is a metal-oxide semiconductor field-effect transistors.
 7. The electronic ballast of claim 1, wherein the first switch and the second switch are electrically connected to a controller, the controller providing switching signals to the first and second switches.
 8. The electronic ballast of claim 7, wherein the switching signals provided by the controller switches the first and second switches off and on in a repeating cycle, the first switch switched on while the second switch is switched off and the first switch switched off while the second switch is switched on.
 9. The electronic ballast of claim 8, wherein the voltage across the first and second switches is substantially constant.
 10. The electronic ballast of claim 9, wherein a current in the first winding provides a charging of the capacitor when the first switch is switched off and a current in the second winding provides a charging of the capacitor over the interval that the respective switch is turned off 